In back-end semiconductor assembly processes, thermosonic ball bonding is by far the most demanding. To make an interconnect between a semiconductor device (commonly called “die”) and its substrate (“leadframe”), very thin wire, commonly gold wire of between 20 to 75 microns in diameter, is bonded, first onto a pad on the die and then onto the corresponding lead of the lead-frame on which the die is mounted. This process continues until all the pads and corresponding leads are connected. One leadframe may have anywhere from less than ten to a few tens of dies. In order to bring successive dies into a bonding area, a linear indexing system (“indexer”), is often used to index the leadframe with high precision. The indexer may essentially comprise a horizontal linear motor driven servo controlled stage on which are mounted two index clamps, one each for indexing leadframes near the input and output sections of a wire bonding machine respectively. Each index clamp has an actuator to control the clamping and de-clamping of the leadframe.
As requirements of both speed and accuracy of wire bonding grow more stringent, all critical subsystems of the wire bonding machine, including the indexer and the index clamp need to be made increasingly more robust, reliable and accurate. For the index clamp, this translates to a requirement of higher clamping force without undue increase in clamp mass. A clamping force of 40N may be required for the clamp for which the cycle time can vary anywhere from 2 to 60 seconds, depending upon the die size which in turn determines the number and length of the wires to be bonded. Correspondingly, the duty cycle can vary widely from 10 to 80%. This is so because at times, the index clamp needs to remain clamped near an already bonded semiconductor package even as bonding takes place on another die which is upstream on the same leadframe. This helps to guard against any possible damage to the already bonded wires due to strong vibrations of the work-holder during the bonding process.
Another requirement of the index clamp is that both its upper and lower jaws should preferably be actuated during the clamping and de-clamping actions. In earlier generation wire bonding machines, even as a leadframe was passed through the work-holder channels, it always rested on the lower jaw of the index clamp. Only the upper jaw was kept movable in order to effect the clamping action.
A conventional solenoid actuated clamp comprises a fixed solenoid and a movable solenoid plate which is free to rotate about a pivot. When the solenoid is energized with an electric current, it attracts the solenoid plate which rotates about the pivot to move a clamping jaw to clamp a substrate. The solenoid may provide a clamping force of about 50N. De-clamping of the substrate is effected by turning off the solenoid current, whereat a return spring which was compressed during the clamping action, moves the clamping jaw in an opposite direction to open the jaw.
There are several drawbacks of this design. First, since the solenoid is a highly non-linear device, its force increases rapidly as the solenoid plate approaches the solenoid. This leads to an undesirably high impact on the substrate during clamping. Secondly, since the force falls off rapidly with an increase in the gap between the solenoid and solenoid plate, there is a limit to the stroke of the plate displacement and thus a limit to the range of thickness of substrates which can be clamped.
Thirdly, the prior art affords no active control during de-clamping, but has to depend upon the return spring. In order to enable quick de-clamping action, the spring force has to be fairly high. This consumes part of the solenoid force, thus leading to a smaller clamping force than would have been possible without the spring. Thus, in order to effect a required level of clamping force, the solenoid has to be larger in size.
An improved actuation mechanism is described in U.S. Pat. No. 6,783,052 entitled “Clamp Actuation Mechanism”, which discloses an actuation system to actuate clamping and de-clamping of a clamping mechanism. The actuation system comprises a variable reluctance actuator such as a solenoid that is operatively coupled to a linear induction actuator such as a cylindrical linear motor that cooperate to provide an actuation force to the clamping mechanism. The cylindrical motor comprises a cylindrical magnet creating a magnetic field within which a coil is movable by electromagnetic interaction.
Whilst such a design can avoid imparting too much impact onto the substrate during clamping, one problem is that the cylindrical voice coil motor that is used provides a relative lower clamping force than a solenoid of the same weight. Thus, it is less effective in providing a high clamping force. A lower clamping force makes it more likely that a substrate may slip during indexing motion.